This invention relates to optical fiber communication systems and, in particular, to a multimode fiber communication system employing offset pinhole illumination of the multimode fiber for enhanced bandwidth transmission.
Multimode fiber communication systems are widely used for short distance systems such as local area networks (LANs) used to wire campuses, offices, factories and other buildings.
A shortcoming of multimode systems is the tendency of light launched in one mode to couple into other modes. Since each different mode has slightly different propagation characteristics, this coupling can spread propagating pulses in time (modal dispersion), limiting the useful bandwidth-distance product of the system.
One approach to enhancing the bandwidth of multimode systems is to illuminate the multimode fiber with a smaller single mode fiber at a position offset from the center of the multimode fiber core. See International (Patent) Application No. WO97/33390 published Sep. 12, 1997 and entitled xe2x80x9cMultimode Communications System,xe2x80x9d which is incorporated herein by reference. Such systems referred to as offset-launch mode-conditioning patch-cord assemblies, strongly excite mid-order modes of the multimode fiber but only weakly excite low order and high order modes. Since the mid-order modes have similar propagation constants, the modal dispersion is small compared to an excitation of all modes. This offset illumination permits an- increase in the effective bandwidth of a multimode system.
A difficulty with the patch-cord assembly is that it requires precise alignment and joinder of a tiny single mode fiber in relation to both the illumination light source and the small core of the multimode fiber. Such precise alignments and junctions are difficult, time-consuming and expensive to make. Accordingly there is a need for an improved arrangement for providing offset illumination of a multimode fiber.
In accordance with the invention, a multimode optical fiber communication system is provided with offset illumination by disposing an optical pinhole adjacent an end of the multimode fiber core and offset from the center of the core. The pinhole permits direct offset illumination without the difficulty and expense of a conventional patch-cord assembly.